It is common to use Pulse Width Modulation (“PWM”) to control the speed of direct current, dc, motors that are utilized with fans in computer applications. Advantageously, PWM control provides for accurate motor speed control.
To reduce fan generated noise and also to reduce power consumption, most modern computers utilize fan controllers to respond to temperatures within the computer and to vary the speed of a fan to provide enough cooling to maintain a desired temperature.
Typically, the fan motor is not operated until such time as there is a need for cooling. It is desirable to be able to initially operate the fan at a low speed to provide cooling of slight temperature increases. However, the static friction of any motor is much higher than dynamic friction of the rotating motor. To assure quick starting of direct current fan motors, it is common to “kick-start” the motor by applying a dc current pulse of fixed time duration to get the rotor in motion and then switch to PWM signals that represent the desired motor rotational speed. The optimum period of the kick-start pulse is determined by the motor and the load inertia and is best determined experimentally as that which gives the most reliable starting performance. In many applications, the fan motor is run at full power for one second to assure that the static friction has been overcome and the motor is rotating.
The drawback to the kick-start is that fan is initially run at full speed and then slows down to the needed speed.
It is highly desirable to provide a motor controller arrangement that minimizes initial fan roar that occurs during timed kick-start operation.